Multiplex receiving system



Feb. 16, 1943- A. D. BLUMLEIN MULTIPLEX RECEIVING SYSTEM Original Filed-Nov. 6, 1936 gl Owrur L l F klld I 1 L l I M i 33 Eu:g 2 INVENTOR ALANvows/2 BLUMLE/N 33 ATT o RNEY Patented Feb. 16, 1943 I 2,311,021MULTIPLEX nsomvme SYSTEM Alan Dower Blumlein, Ealing, London, England,

assignor to Electric & Musical Industries Limited, Hayes, Middlesex,England, a British com-- Original application November 6, 1938, SerialNo. 109,456. Divided and this application July 19,

1939, Serial No. 285,276.

November 14, 1935 In Great Britain 4 Claims. (01.179-15) The presentinvention relates to multiplex signalling systemannd more particularlyto receiving apparatus as used in such systems. This application is adivision of my application Serial No. 109,456, filed November 6, 1936.

Multiplex systems are known in which a plurality of telegraph channelsare obtained through a single circuit. The circuit may be a land-line ora radio link. In these systems it is usual to employ mechanicaldistributors to connect the circuit in turn to each telegraph instrumentat the transmitting and receiving point simultaneously. The whole cycleof connection of the line to all the receiving instruments on thecircuit must occupy a time less than the time occupied by one signaldot.

The present invention deals with systems in which telephone frequencycircuits may be connected in a multiplex manner through a circuit suchas a radio link or high frequency cable intended to cover a wide rangeof frequencies.

In such systems, on account of the comparatively wide band offrequencies required for each channel mechanical distributors arunsuitable.

It is an object of the present invention to provide improveddistributors capable of high speed raphy, said system comprising meansfor feeding signals from a plurality of channels through a singlecircuit, the arrangement being such that,

in operation, there is transmitted through said eircuit a train ofelementary signals interspersed with synchronizing signals having anamplitude outside the amplitude range of said train of signals, eachelementary signal being representative of the signal in one of saidchannels, and means being provided for utilizing said synchronizingsignals to control switching devices for connecting said channels inturn to said circuit.

The wave transmitted through the circuit may comprise a plurality ofgroups of signals, each group comprising a plurality of trains ofelementary signals, and each elementary signal being representative ofthe signal in one of the channels. Each train then comprises the signalsfrom a set of channels and successive trains comprise the signals fromdifferent sets of channels. group of signals includes an elementarysignal from every channel; the trains within a group are separated fromone another by synchronizing signals and the groups are separated fromone another by further synchronizingsignals of Each different durationor different amplitude, or of both different duration and difl'erentamplitude from the synchronizing signals separating the trains within agroup.

In one arrangement according to the invention, the switching meanscomprise a cathode ray tube in which an electron beam is caused to scana screen having a number of targets associated with the incoming oroutgoing channels. The tube thereby serves as a distributor. Further,according to the invention, there are provided means for feeding a partof the signal from one channel into a succeeding channel at thereceiving end of the circuit for the purpose of neutralizing or reducingcross-talk which may be present due to distortion of signal wave-form.

In order that the invention may be more clearly understood, and readilycarried into effect, several embodiments thereof will now be describedwith reference to the drawing wherein:

Figure 1 shows the wave form of a multiplex signal which is obtained inone arrangement according to the invention;

Fig. 2 illustrates the wave form of a multiplex signal which is obtainedin another arrangement according to the invention;

Fig. 3 shows a circuit for reducing or neutralizing cross-talk betweenchannels;

Figs; 4 and 5 show arrangements employing cathode ray tubes at thesending end and receiving end, respectively; and

Fig. 6 shows a construction of signal plate suit able for use in thetubes of Figs. 4 and 5.

It is assumed that it is desired to transmit signals from a plurality ofchannels (telegraph, telephone or other signalling channels) through asingle circuit which may comprise a radio link or a cable capable ofhandling signals covering a wide range of frequencies. The circuit isassociated at its two ends with distributors which are adapted tooperate synchronously to connect the transmitting and receiving channelssuccessively to the circuit. Thus, if there are 11 channels, thedistributors first connect the first channel to the circuit for a shortperiod of time, thus allowing a signal to pass in this channel; thedistributors then switch over to the second channel to allow a signal topass in the second channel and then to the third, fourth, etc., up tothe nth channel. The distributors then switch back to the first channeland the process is repeated continuously. The frequency at which thedistributors make complete cycles must be at least as high as theminimum frequency which causes no deleterious effect on the signalsbeing transmitted. Thus, if the channels are telegraphic, the frequencyat which the distributors complete a cycle of change must be greaterthan the reciprocal of the time occupied by one signal dot.

Fig. 1 shows the wave form of a signal in the single connecting circuitin a system according to the invention. The wave comprises a series ofuniformly spaced synchronizing pulses of which two are indicated by thereferences and 0 in the figure. The zero line of the signals is shown bythe line at, 0:. The signal from the first channel is indicated at I,that from the second channel at 2, that from the third channel at 3, andso on for the remaining channels. After the synchronizing signal 0', thechannels are again connected in turn to the circuit and the portions ofthe composite signal due to the signals in the first, second and thirdchannels are indicated at l', 2 and 3, respectively. It will be seenthat, during the interval between the corresponding points in theseconsecutive cycles of the distributors, the signal value in channel Ihas become less positive, that in channel 2 has changed from a negativevalue to a positive value, whilst that in channel 3 has remainedunchanged. By employing suitable circuits to separate the pulses of saychannel I at the receiving end, and rectifying these pulses, they may becaused to produce a signal of a wave-form substantially corresponding tothe wave-form of the signal in channel l at the sending end. Similarlyfor the other channels. If the synchronizing pulses occur at a frequencyof 10,000 per second, then signals embracing frequencies up to nearly5,000 cycles per second can be transmitted through the circuit on eachchannel.

If it is assumed that the multiplex signal of Fig. 1 can be transmittedwith sufficient sharpness through a circuit capable of handling afrequency range extending to one-half the number of pulses transmittedper second (the number of pulses per second is equal to the product orthe frequency of the synchronizing pulses and the number .of channels),then a frequency range of 5,000 n cycles per second will be required for11. channels each requiring the transmission of frequencies up to 5,000cycles per second. It will be seen that this is exactly equal to theminimum frequency range required by a multiple carrier signalling systemradiating n single side-band modulated carriers, each having a side-bandWidth of 5,000 cycles per second, it being assumed that the channels areclose packed and are separated by infinitely sharp filters at thereceiving end. In practice the multiplex signal requires a greaterfrequency band than that mentioned above but a corresponding multiplecarrier system also requires a range greater than the theoreticalminimum value mentioned above. I

In the multiplex system considered above the synchronizing pulses 0, 0are employed for maintaining synchronism between the distributors at thetwo ends of the connecting circuit. Such an arrangement is quitesatisfactory provided that the number of channels is not too great, forexample, not greater than 20. If the number of channels (11) is muchgreater than 20, then errors may be introduced in dividing the intervalsbetween successive synchronizing pulses into n equal periods. Theseerrors will not, in general, be of equal magnitude and in the same senseat the two ends, and the result may be incorrect connections to thechannels. For example, it might happen that when the 31st channel wasconnected to the circuit at the transmitting end, the 30th channel wasconnected to the circuit at the receiving end. Even if the error islnsum- 'cient to cause a signal pulse to be wholly fed to an incorrectchannel, the error may be sufficient for a part of a. pulse to be fed toan incorrect channel.

If a large number of channels is to be employed, for example 200, thesignals are preferably divided up as shown in Fig. 2. In this figure,rectangles L, M, N are employed to denote trains of signals. The train Lcontains signals from channels 1, 2, 3 p, the train M contains signalsfrom channels p+l, p+2, p+3 2p, and so on. Thus if there are q trainseach deriving signals from different channels, a number of channelsequal to the product p.q. may be represented in the composite signal.After the end of the qth train, a repetition of the sequence begins. Thetrains L, M, N are separated from one another by synchronizing pulses 0similar to those of Fig. 1. A sequence of q trains of signals which willbe termed a group of signals is separated from the succeeding group by agroup synchronizing signal which diflers from train synchronizingsignals 0. Thus, the group synchronizing signal 00 shown before train Ldiffers from train synchronizing signals 0 in that the group signal hasa longer duration than the train signal.

When the group signals are employed to operate a primary distributor andthe train signals are used to operate a. secondary distributor, it ispossible to connect 400 channels in rotation without dividing anyinterval between synchronizing signals into more than 20 parts. Thus, ifa group comprises 10 trains ((1:10), each train representing 20 channels(19:20), then the 199th channel is selected by counting from groupsynchronizing signal 00 up to the 9th train and then counting up to the19th signal in this train. If it is desired to transmit telephonyinvolving frequencies up to about 5,000 cycles per second, then thegroup synchronizing signals must have a frequency of 10,000 per second;in the example considered in which there are 10 trains per group, thetrain synchronizing frequency is 100,000 per second.

It will be observed that the group synchronizing signal 00 functions forgroup L as a train synchronizing signal. If desired, a trainsynchronizing signal of normal form may be inserted between the signal00 and the beginning of the group L. In another arrangement, either theleading or trailing edge of signal 00 is timed to take the place of atrain synchronizing signal. In yet another arrangement, the groupsynchronizing signal 00 is broken into parts in the manner known forframe synchronizing signals in television systems, one of these partsbeing used as a train synchronizing signal. It will be seen that thegroup synchronizing signals 00 and the train synchronizing signals 0bear a close resemblance to the frame and line synchronizing signalsused in many television systems.

For multiplex telephony by the method Just discussed, group and trainfrequencies are much higher than the frame and line frequencies commonlyused in television systems. However, by using synchronizing frequenciescomparable with those employed in television, it is possible to realizehigh speed multiplex telegraphy. By employing somewhat lowersynchronizing frequencies, multiplex telegraphy over a circuit with aband width of the same order as that of ordinary telephone circuits maybe obtained. Such a multiplex system may be used for remote contropurposes whereby each channel controls the operation of a device such asa switch or a rheostat. Again, the system may be used for remotemetering where the impulses are proportional to the readings of current,power or other measurable quantity in various circuits.

It will be seen that for the successful handling of signals such as areshown in Figs. 1 and 2, it is necessary for the transmission circuit tohave a uniform frequency response over the required range and also to besubstantially free from phase distortion. The requirements of thetransmission circuit are therefore the same for the above purpose as fora television link. Sometimes the low frequency components are nottransmitted. That is to say, they are fed to the transmission circuit,or they may be lost either in the transmission circuit or at thereceiving end. It is possible, however, to re-establish the lowfrequency components at or before the distributor at the receiving endwith reference to the'peaks of the synchronizing signals or withreference to some other recurrent fixed amplitude (for example, the zeroperiod shown at :u in Fig. 1) in a manner well known in televisionsystems and generally referred to as D. C. r'e-insertion.

The possibility of reinsertion of D. C. allows low frequency componentsto be neglected at the receiving end so that noise due to induction frompower circuits etc., can be lessened in its effects. This isparticularly the case where a very high synchronizing frequency isemployed such as that described for the transmission of 10 trains ofsignals with a group frequency of 10,000 per sec. employing a trainsynchronizing frequency of 100,000 per sec. Such a signal may betransmitted over a concentric single core cable and the low frequency,subjected to induction, may be neglected, since the lower frequenciesand the D. C. may be reinserted with reference to a signal occurring100,000 times per sec. At the higher frequencies the concentric cable isnot subject to induction owing to the thickness of its sheath.

Fig. 3 shows schematically a method of correct ing cross-talk due to badpulse shaping. If the channel does not pass very high frequencies, thepulse representing th signal for any one elementary channel persists inthe time allocated for the next channel, or even in a bad case into thenext channel but one. Such persistence may take the form of a gradualdecay of the pulse, which adds a signal in the same sense to thefollowing channel. Alternatively, the persistence may consist of anovershoot in the opposite direction, which adds an opposite signal tothe succeeding channel. I

Fig. 3 also shows the anode connection for two hexodes arranged as inFig. 5 of my Patent 2,172,354 (of which the instant application is adivision) except that the outputs are taken away through capacitativeconnections 20. To points 20 are coupled triodes 2|. Triodes 2| serve toamplify the signals for the purpose of providing cross-talk correction.The amplifying triodes have resistance potentiometers in both anode andcathode circuits and cross-connecting leads it pass to the succeedingstage and may then be connected across either the cathode or anodepotentiometers of the valves 2|. These connections feed current into theoutput of the succeeding stage and can be set to neutralize thecross-talk therein by suitable positioning of the switches 23 and.adjustment of the appropriate potentiometer. It will be noted thatcross-talk is to a small extent fed on to the next stage but one, sothat assuming a logarithmic decay or a logarithmic decaying series ofover-shoots, cross-talk correction can be achieved for a large number ofstages. As an alternative, a small telephone frequency coupling may bemade between the anode of one valve and the grid of the next, whichcorrects cross-talk satisfactorily enough for most purposes. Thecross-talk correction circuits need not pass dot frequencies from theline, but should be flat for the telephone frequency range.

Figs. 4 and 5 show arrangements for sending and receiving signals of thetype shown in Fig. 2 by means of cathode ray tubes. The evacuated glassenvelope is not shown in either Fig. 4 or Fig. 5. In both figures thelead 24 is connected to a target 25 on the signal plate 26 which isscanned by a beam of electrons produced by a gun assembly 21. This beamis caused to scan the signal plate 26 by means of two saw toothoscillation generators, 28, the outputs of which are applied toelectrostatic deflecting plates 30, 3|. In the case of both tubes, thetarget 25 i one of a number of targets arranged in rows on the plate 26.These targets may be constructed as shown in Fig. 6, where a metal plate26, which may be the envelope of a cathode ray tube, has

holes in it through which are pushed metal plugs 33 insulated from thesignal plate 26 and held gas-tight by glass beads 34. The formation ofsuch a plate requires glass with a. coeflicient of expansion similar tothat of the metal employed. In a'transmitting tube shown in Fig. 4, awire mesh screen 31 is interposed between the gun 21 and the signalplate 26 containing the targets, which screen is held positive withrespect to the gun cathode 35. The signal plate 26 is held slightlynegative with reference to the gun cathode 35 and the mean potential ofthe target is held slightly positive with respect to th gun cathode 35.When the beam is directed at a given target, a fraction of the electronsare turned back from the target to the grid meshwork in front of it.This fraction turned baci; depends on the potential of the target, whichdurin the moment of scanning is that of the telephone channel which isconnected to the target in question.

A small condenser 38 is shown as representing the capacity to earth ofthe target 25, which should be large enough to hold its potential steadyduring the instant of scanning. The currents to the screen 31 are passedthrough a resistance 38, the voltage across which is amplifled at 39 andpassed to the cable or radio link by a lead 40. The scanning pulses aregenerated by an oscillator ll and pulse generator 42 as for television,blackout being provided to tum the cathode ray beam of! during returnstrokes. Synchronizing pulses are also generated and mixed in with theline signals. The wave-form obtained is similar to that shown in Fig. 2if an area is scanned, or as shown in Fig. 1 if only one line of targetsis scanned. In the latter case only line scanning is provided and oneset of deflecting plates may be dispensed with.

As an alternative to taking the signals from i the screen 31, they maybe obtained from the metal plate 28, the signals being fed from theelements through capacities between elements and this plate. whichcapacities may be the inherent capacities formed in the construction;the

' telephone frequency currents being fed to the duced simply by shuttingthe beam off, since a limiting signal in one direction is produced bysuch switchin of the beam.

Fig. 5 shows a circuit .for use at the receiving end, a signal platesimilar to that at the transmitter being used, except that the grid infront of the targets is omitted. The line signals coming in are passedto saw-tooth oscillation generators 28, 29 which produce saw-toothsynchronizing signals as in a television receiver. Amplified linesignals are passed to the control electrode 43 of the cathode ray gunwhich modulates the beam in accordance with the line signals. The beamimpinges on to the target which is held positive with respect to thesecond anode M to prevent the flow of secondary emission current. Thecurrent in the target is therefore proportional to the beam current, anda direct coupling is taken from the target to the telephone circuit 45,with the interposition, if necessary, of a filter to smooth out thepulses. As before, for waves of the form shown in Fig. 2', a

complete area is scanned; for those of the form of Fig. 1, only a lineis scanned. In both the transmitting and receiving tubes the scanningvelocity must be maintained accurate to within one part in the number ofelements in a train or the number of trains in a group.

The cathode ray transmitting and receiving apparatus may be usedinterchangeably with a delay network type as shown in the earlierdrawings; thus a cathode ray transmitting tube may be used with a delaynetwork receiver and vice versa.

The transmission of signals here described requires a transmission-linksuitable for facsimile or television signals, in that substantially nophase distortion is tolerable within the working frequency range. On theother hand, curvature distortion due to amplifying tubes, etc. does notproduce cross-talk in the channels as in a multiple carrier transmissionsystem.

It will be understood that the scope of the invention is not limited tothe arrangements shown in the drawing, and that these are by way of illustration only.

It will be understood by those skilled in the art that bridge circuitsof known type, including contact rectifiers, may be used, the signalamplitude modulating the pulse amplitude being passed through the bridgecircuit. Tests have shown that it is possible to transmit a telephonefrequency range which extends up to half the cycle frequency. Thus ii apulse representing the message of one particular channel is sent 7,000times per second, then frequencies up to 3,500 can be transmitted onthis channel. Any attempt to transmit higher frequencies than 3,500leads to the production of unwanted difference frequencies. If suchfrequencies exist in the telephone channel, it is necessary to insert asimple filter to cut out frequencies above half the pulsing frequency.

The systems here described are very suitable for operation over cableswhich are built to constant attenuation and velocity.

What is claimed is:

1. In a multiplex system which is receptive of a succession of trains ofelementary signals, each elementary signal being appropriate to adifferent channel, and there being a group of channels appropriate toeach train, said elementary signals being restricted to a predeterminedamplitude range and being interspersed with synchronizing signals theamplitude of which exceeds successive elementary signals into dii'ierentones of said receiving circuits in turn, and means responsive to saidsynchronizing signals when they possess another characteristic forcausing said channel distributor to switch successive trains ofelementary signals into different groups of said receiving circuitswhich are respectively appropriate to different ones of said groups ofchannels.

2. In a multiplex system in which a plurality of groups of signals isreceived throuzh a single circuit, each group comprising a plurality oftrains of elemental signals, separate signals being respectivelyappropriate to difl'erent channels, said signals being restricted to apredeteh. mined amplitude range, the trains within a group beingseparated from one another by synchronizing signals of onecharacteristic, and successive groups of trains being separated from oneanother by synchronizing signals of a diflerent characteristic, thesynchronizing signals of both characteristics being oi an amplitudeoutside said predetermined amplitude range, a. plurality of receivingcircuits each. appropriate to a different multiplex channel, acontinuously operable channel distributor for switching successiveelemental signals cyclically into diflerent ones of said receivingcircuits, means responsive to the first said synchronizing signals forcorrecting the phase of said distributor with respect to each said trainof signals, and means responsive to the second said synchronizingsignals for correcting the phase of said distributor with respect toeach said group of signal trains.

3. In a system for receiving multiplex signals which have beentransmitted in groups through a single circuit each group comprising aplurality of trains of elementary signals representative of signals inseparate channels, means for amplifying said signals to within apredetermined amplitude range, a cathode ray tube distributor having amosaic of target electrodes by which said signals after amplificationare successively allocated to separate receiving channels, phasecorrecting means for said distributor, the last said means beingresponsive to synchronizing signals of two types wherein those of onetype are of different duration and amplitude from those of the othertype and all synchronizing signals are amplitudinally distinguishablefrom said elementary signals, means responsive to synchronizing signalsof the first type for operating the phase correcting means so as tomaintain suitable separation between said trains of signals, and meansresponsive to synchronizing signals of the second type for operating thephase correcting means so as to maintain suit able separation betweenthe groups of signals.

4. A multiplex. receiving system comprising means for selecting andamplifying to within a predetermined amplitude range elementaryintelligence-bearing signals, means for selecting synchronizing signalswhich have been interspersed between successive trains of the firstsuccessive groups of said trains, deflecting circhanneis, line scanningmeans operable by one of said deflecting circuits at the frequency ofsaid train-interspersed synchronizing signals, and frame scanning meansoperable by the other of said. deflecting circuits at the frequency ofsaid group-interspersed synchronizing signals. ALAN DOWER BLUMLEIN.

